Glycinonitrile is an important intermediate in the preparation of alkali metal glycinates and glycine. As such, impurities in the glycinonitrile affect the purity and yield of the glycinates or glycine, often deleteriously.
Conventional glycinonitrile preparation requires essentially stoichiometric amounts of hydrogen cyanide and formaldehyde, but a minimum of about 200% excess ammonia. Several different routes for the preparation of glycinonitrile are known. For example, glycinonitrile can be prepared by the ammonolysis of glycolonitrile; by the reaction of an alkali metal cyanide, formaldehyde and an ammonium halide in the presence of acid; or by the reaction of chloracetonitrile and ammonia.
U.S. Pat. No. 3,167,582 discloses a process for the preparation of glycinonitrile by reacting hydrogen cyanide, formaldehyde and ammonia under critically controlled reaction conditions. Thus the mixing of the reactants, the mole ratio of the reactants, the temperature and pH of the reaction, and the residence time of the reactants within the reactor are parameters that are critically controlled so as to obtain glycinonitrile in high yields in a continuous manner.
The foregoing processes suffer from various drawbacks, such as low purity, resulting in low purity derivative products, and low product concentration, resulting in low production capacity and labor-intensive procedures.
A major drawback of present commercial processes for the preparation of glycine is the generation of waste crystal liquor. This liquor purges from the system the iminodiacetic acid (IDA) and other impurities which largely come from the impure glycinonitrile which is fed into the saponification step to produce the Na glycinate from which the glycine acid is derived. The substitution of NH.sub.3 by cyanide and formaldehyde is never exclusively on a 1:1 molar basis to produce only glycinonitrile. Disubstitution always occurs to produce HN(CH.sub.2 CH).sub.2, or IDAN. Only by using massive excesses of NH.sub.3 can this by-product formation be reduced to low levels. At a mole ratio of about 3:1 NH.sub.3 :(HCN+CH.sub.2 O), the mole % conversion of (HCN+CH.sub.2 O) to glycinonitrile is 85-95%. Most of the other 5-15% goes to IDAN.
Because the nitrile is a solution, no purification by crystallization can occur. Typically the glycinonitrile solution is directly saponified in NaOH solution: the glycinonitrile to Na glycinate, and the IDAN to IDANa.sub.2. The solution of Na glycinate and IDANa.sub.2 is subjected to crystallization at pH about 5-7, which is at or near the isoelectric point of glycine acid. At this pH the IDA exists as its very soluble mono Na salt (IDAHNa). The IDA must be purged from the system as a waste liquor containing glycine, IDAHNa, and other impurities, thereby generating a substantial waste which is responsible for the relatively low yields of glycine from the original HCN.